The invention is directed to medical devices implantable into the body of a living being having means for stimulating a physiological function of the being with adjustable stimulation intensity, comprising a sensor for forming a signal corresponding to the physical activity of the living being, means for implementing an algorithm for calculating a stimulation intensity adapted to the physical activity of the living being, and adjustment means that sets the stimulation intensity in view of the algorithm. The invention is also directed to a method for adjusting the stimulation intensity of the implantable device.
As used herein, the term "stimulation intensity" is understood to be comprehensive and to include duration, frequency, repetition rate, amplitude, etc. with which, e.g., means for stimulation are activated. Thus, the term "stimulation intensity" means any combination of the above-listed parameters.
Implantable medical devices of the type described above allow a living being, in whom they are implanted, to lead a normal life insofar as possible by providing necessary stimulation of a malfunctioning physiological function. The stimulation is provided with an intensity that depends upon the physical activity of the living being, the stimulation intensity corresponding as much as possible to that intensity that would be present if the living being were not dependent upon the artificial stimulation of the physiological function by the implantable medical device.
In U.S. Pat. No. 4,428,378, the teachings of which are fully incorporated herein by reference, there is disclosed an implantable medical device of the type discussed above. The disclosed device is an implantable heart pacemaker. The device includes a piezo-electric pressure sensor integrated therein that registers mechanical vibrations in the body of the being arising from movement of the muscles and the like during physical activities of the being, these mechanical vibrations propagating as pressure waves, and that converts the vibrations into a correlated physical activity electrical signal. The stimulation intensity, i.e., the stimulation frequency with which the heart pacemaker stimulates the heart given the absence of natural heartbeats, is calculated with reference to the correlated physical activity. The calculation is made according to a predetermined algorithm. The stimulation intensity is then set via adjustment means.
The algorithm used is based on the characteristics of an average patient, on the average coupling relationships of the piezo-electric sensor to the body of the patient, and on the average manufacturing tolerances with respect to the heart pacemaker, particularly with respect to those components that are used in the formation of the electrical signal correlated to the physical activity. This means that the stimulation frequency that is set and calculated according to the algorithm only rarely coincides with the heartbeat frequency with which the heart of the patient would be at spontaneously given a physical activity. In the majority of cases, the stimulation frequency that is set for a defined physical activity can deviate greatly from the spontaneous heartbeat frequency that the patient would otherwise have experienced given the physical activity. Thus, it can occur that the calculated stimulation frequency can correspond well to the patient's requirements for a time following implementation of the heart pacemaker, but otherwise deviates more and more from the spontaneous heartbeat frequency with which the heart of the patient would beat due to, for example, tissue growing over the piezo-electric sensor.